The current invention is directed to the analysis of fluids. More specifically, the current invention is directed to the compositional analysis of fluids, such as fluids produced by oil wells, that contain constituents that fluoresce and/or absorb radiation, such as near-infrared radiation.
Monitoring of the fluids produced by an oil well, such as compositional analysis, provides valuable information that allows production to be optimized. In the past, such monitoring was performed by analyzing fluid samples brought to the surface, typically using techniques such as ultraviolet-visible (UV-Vis) absorbance spectroscopy, infrared (IR) absorbance spectroscopy, UV fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry, and gas chromatography.
Unfortunately, these traditional surface fluid analysis techniques are of limited value in many wells created using modem drilling and production methods. This is so because modem methods often result in the creation of complex and/or difficult to monitor wells, such as multizone, horizontal, or multilateral wells. In such wells, fluid produced from different zones of the well may be combined downhole so that the fluid discharged at the surface is a mixture. Analysis of this mixture provides little information concerning the component of the fluid production associated with any of the individual zones of the well, which is necessary to maximize the overall production of oil while minimizing the production of water. For example, if one zone were producing fluid with a high water content, a control device could be operated to limit or cease production from that zone. Subsurface monitoring at the source is also advantageous where accurate knowledge of various field""s production rates are required. For example, in subsea applications, fluid from different reservoirs may be combined at a subsea manifold. Production monitoring at this point is desirable to allow the operator to make control decisions regarding individual wells.
Another disadvantage of surface techniques is that they analyze the fluid after it has flowed through a long production tubing, which can alter the phase properties of the fluid (e.g., induce slugging). By contrast, downhole analysis provides real time data on conditions occurring at the point of production in the well.
Consequently, it would be desirable to provide a system and method for analyzing fluid produced in each individual zone of the well prior to intermixingxe2x80x94that is, in a downhole environment.
The ability to remotely sense the presence of certain fluids, such as oil, in a flowing stream, is also desirable in situations other than in oil wells. For example, it is sometimes desirable to determine when a fluid, such as discharge water, that should not contain oil has become contaminated with oil. Consequently, it would be desirable to provide a system and method for analyzing the presence of certain fluids in a flowing stream.
When light strikes a fluid, several phenomena may occur. A portion of the light may be reflected from the surface, while another portion will enter the fluid. The portion of the light entering the fluid may be transmitted through the fluid or subjected to scattering or absorption. Very often, all of these mechanisms occur simultaneously.
Light may scatter as a result of several different mechanisms. If more than one phase is present in the fluid, light will be scattered by reflection and refraction at the interfaces between the phases. Scattering will also occur as a result of the Rayleigh mechanism. Light scattered by the Rayleigh mechanism has the same wavelength as that of the incident light. In some substances, such as oil, scattering also occurs by the Raman phenomenon. Raman scattering produces extremely low intensity light (relative to the intensity of the incident light) having wavelengths both above and below that of the incident light, so that even monochromatic light yields scattered light in a range of wavelengths. Thus, when analyzed by a spectrograph, Raman scattering produces lines on both sides of the Rayleigh line that are a characteristic of the substance and upon which the light is incident can be used its composition.
Previously, it has been proposed to use Raman scattering to determine the composition of certain types of hydrocarbons in refineries, such as disclosed in U.S. Pat. No. 4,620,284 (Schnell et al.). However, Raman analysis cannot be used to determine the composition of a mixture of crude oil and water, such as that flowing through a well, for two reasons. First, crude oil is highly fluorescent so the fluorescent radiation, which has a longer wavelength than the incident light, would overwhelm the Raman signal even when using a near infrared excitation source. Second, the light emitted as a result of Raman scattering is too low in intensity to be transmitted to the surface for analysis, while the down hole environment is too harsh to permit the use of the sensitive equipment, such as a spectrograph and charged couple device, necessary to conduct a Raman analysis down hole.
In addition to scattering, a portion of the light entering the fluid may be absorbed. The amount of light absorbed at a given wavelength is a characteristic of the substance. Therefore, the constituents of a substance can be determined by comparing the spectrum of the light directed into the fluid with that of the light that has been transmitted through it so as to determine the spectrum of the light absorbed by the fluid. This spectrum may be expressed, for example, as xe2x88x92log10 of the ratio of the light directed to the fluid and the light transmitted through the fluid. Although compositional analyses using absorption have been proposed in the past, they suffer from the fact that the intensity of the light transmitted through the fluid depends on scattering, as well as absorption. Whereas absorption is primarily a function of the constituents of the fluid, scattering also depends on the physical form of those constituents. For example, in an emulsion, such as a mixture of water and oil, the more finely dispersed the oil droplets the greater the scattering. The increase in scatting associated with the reduction in droplet size will reduce the intensity of the transmitted light, despite the fact that the composition of the fluid, in a quantitative sense, has remained unchanged. Scattering can, therefore, lead to significant errors in systems measuring the absorption spectra of the fluid.
U.S. Pat. No. 4,994,671 (Safinya et al.) discloses a method for analyzing the composition of fluid in a well by suspending within the well a tool that contains a spectrograph and an incandescent tungsten-halogen lamp. The lamp is characterized as being relatively bright in the 1000 to 2500 nm range and down to about 500 nm and having acceptable emissions from 350 to 500 nm. The lamp directs light onto a sample of fluid that is admitted into the tool. Different sections of a fiber optic bundle receive the light transmitted across the fluid sample, as well as the light back-scattered from the sample. The spectra of both the transmitted light and the back scattered light are measured by a spectrograph and the data are digitized and transmitted electronically to a computer at the surface. Two absorption spectra for the fluid are determined by dividing the transmitted light spectrum and the back scattered light spectrum by the spectrum of the source light. If the fluid is sufficiently transparent to transmit an adequate amount of light through it, Safinya recommends the use of the transmitted light; otherwise the back-scattered light may be used. The computer determines the constituents of the fluid sample by comparing the transmitted or back-scattered absorption spectra to a data base containing reference spectra for water, gas and various types of oils, and using a least squares or principal component analysis method. Since the spectra may vary with the temperature and pressure, Safinya discloses that in order to obtain an accurate analysis, the data base should contain reference spectra for the various constituents at a variety of pressures and temperatures. Unfortunately, Safinya""s method suffers from a variety of drawbacks that have made it unsuitable for use in practical applications.
First, as indicated in U.S. Pat. No. 5,266,800 (Mullins), the computations necessary to perform the analysis taught by Safinya are computationally intensive and required an extensive data base of spectra for water, gas and oils.
Second, and perhaps more importantly, Safinya does not account for the effect of variations arising from scattering. The flow of a multicomponent fluid (e.g., oil, water and gas) through a production well has very complex multiphase properties. Variations will occur not only in terms of the relative proportion of the constituents but also in multiphase characteristics, such as droplet or bubble size and the composition of the continuous and dispersed phases (e.g., oil and gas bubbles dispersed in water, oil droplets dispersed in gases, etc.). Additionally, there may be particulate matter suspended in the fluid, which can add to the scattering. As discussed above, variations in these physical characteristics of the fluid will cause variations in the intensity of the transmitted or back scattered light that, according to Safinya""s method, will cause an apparent, but erroneous, change in the composition of the fluid. For example, suppose that the spectrum is obtained of a fluid flowing through a well that is initially a 50/50 mixture of oil and water, with the water occurring in relatively large droplets. Further suppose, although this is not by any means to be expected, that comparison to the spectra in the data base using Safinya""s method results in the correct determination of the composition. If the fluid remains a 50/50 mixture but the water and oil become more finely dispersed, the intensity of the transmitted light will decrease at all wave lengths, including the intensity of the light in the wave lengths associated with water, which will be interpreted as a greater absorption in the water-associated wave lengths. This, in turn, will lead to the erroneous conclusion that the concentration of water in the fluid has increased.
U.S. Pat. No. 5,166,747 (Schroeder) recognizes that scattering in Safinya""s method can cause the intensity of the transmitted light to undergo swings so wide that they cannot be handled by the spectrograph. Schroeder""s approach to this challenge was, through an opto/mechanical means, to redistribute the composition of the transmitted light reaching the spectral analyzer. Through optical diffusers or misalignment of the input and output fibers, the spectral analyzer received less directly transmitted light and more forward scattered light. The forward scattered light still indicated the absorbance of the sample, but it is of reduced intensity. The weaker signal was an acceptable tradeoff for signal stability. However, this approach is not feasible where the light source and spectral analyzer are at the surface. In such circumstances, the signal intensity is of paramount concern due to the losses that can occur if the sampling portion of the sensor is many kilometers from the surface. Also, the potential for errors due to scatter will still occur and, perhaps, be even greater than those associated with Safinya""s method because the strength of the original signal is reduced.
It is an object of the current invention to provide a method for determining the concentration of a constituent, such as oil or gas, in a fluid flowing in a remote location, such as downhole in an oil well. This and other objects is accomplished in a method of determining the concentration of at least one predetermined constituent in a fluid flowing through a downhole portion a well, comprising the steps of (i) generating a beam of light, (ii) directing the beam of light into the fluid flowing through the downhole portion of the well so as to cause light to emerge from the fluid, the emerging light having been scattered by the fluid and comprised of components each of which has a different wavelength, (iii) transmitting at least a portion of the emerging light to a location proximate to the surface of the earth, (iv) measuring the intensity of each of at least a portion of the components of the transmitted light, each of the light components in the portion of light components having a wavelength falling within a predetermined range of wavelengths, the light component intensity measurements being conducted at the location proximate the surface, (v) normalizing at least those of the measured light component intensities having selected wavelengths so as to reduce the effect of the scattering of the light components on the measured intensities, (vii) exponentially raising and then multiplying each of the normalized light component intensities at the selected wavelengths by a predetermined weighting factor based upon its respective wavelength, and (viii) summing the weighted and normalized light component intensities at the selected wavelengths so as to calculate the concentration of the constituent.
In one embodiment, the method further comprises the step of determining the weighting factors by (i) directing a calibration beam of light into a plurality of fluid calibration mixtures so as to cause light to emerge from each of the calibration mixtures that is comprised of components each of which has a different wavelength, with each of the calibration mixtures containing predetermined varying concentrations of the constituent, (ii) measuring the intensity of each of the components of the light emerging from the calibration mixtures having a wavelength falling within the predetermined range of wavelengths, (iii) normalizing at least a selected portion of the measured intensities of the light components emerging from the calibration mixtures, and (iv) performing a regression analysis on the normalized intensities of the calibration mixtures so as to determine the weighting factors.
The invention also encompasses an apparatus for determining the concentration of a predetermined constituent in a fluid flowing through a downhole portion a well, comprising (i) means for generating a beam of light, (ii) means for directing the beam of light into the fluid flowing through the downhole portion of the well so as to cause light to emerge from the fluid which light is comprised of components each of which having a different wavelength and that has been scattered by the fluid prior to emerging therefrom, (iii) means for transmitting at least a portion of the emerging light to a location remote from the downhole portion of the well, (iv) means for measuring the intensity of each of the components of the transmitted light having a wavelength falling within a predetermined range of wavelengths at the remote location, (v) means for exponentially raising and normalizing at least a selected portion of the measured component intensities so as to minimize the effect of the scattering to the light emerging from the fluid has been subjected on the component intensities, (vi) means for determining the concentration of the constituent based upon the normalized component intensities.
In one embodiment, the apparatus further comprises a computer, and the means for means for normalizing the selected portion of the measured component intensities and the means for determining the concentration of the constituents comprises software programmed into the computer.